The efficiency in transforming the electrical signal into an acoustic signal at the transmitter site and vice versa at the receiver site should remain constant over a wide temperature range.
With resonant ultrasound transducers their frequency of resonance and their radiation behaviour (radiation lobe) should only vary little with changes in temperature or changing masses at the exposed sound transducer surfaces which are due to raindrops, for example. Building-up of ice layers on the exposed ultrasound transducer surfaces under freezing conditions must not result into a breakdown of the system.
As, however, the period in time of such a freezing situation is unknown, building-up of an ice or rime layer has to be avoided for sure, even in case of high wind velocities.
Icing of ultrasound transducers can effectively be avoided, for example, by sufficiently heating them up, if the surface temperature of the acoustically active surface is kept within the positive temperature range (>0° C.).
All known ultrasound transducers have an electromechanical power converter which is based on an piezo-electric or magneto-dynamic effect, i.e. a transducer by which an electric signal can be transformed into a motion (by means of a change in lengths in case of a piezo, for example) and vice versa.
If such a transducer is used without further measures to produce a sound wave in air, coupling the mechanical performance of the solid body of the transducer having a comparatively high acoustic impedance into the air having a very low acoustic impedance is only achieved with high matching losses.
To reduce the matching losses it is known to use λ/4-layers which are commonly denoted as matching layers and which ideally have an acoustic impedance of the geometric average of the impedances of the solid body of the transducer and of the air.
An optimum impedance match by means of such a matching layer is not possible in practice, as the ideal acoustic impedance mentioned above can not be achieved with any known material or material composition.
The acoustic impedance of a material is the product of the density of the material and of the sound velocity in the material.
This context already indicates that materials or compositions suitable for matching layers normally have a low density.
At the same time, the matching layer must not show a high attenuation of the acoustic wave.
Nearly all materials and material compositions which can be used for matching layers to air only have very low heat conductivity as compared to metals because of their physical properties. The requirements with regard to their acoustic properties are directly opposed to the desired good heat conductivity which is necessary for heating against icing.